What is the difference between var, let and const?

The interview answer has three parts: scope, hoisting and reassignment. Most candidates stop at "const means constant", which is where the follow-up lands.

Scope

function scopeDemo() {
  if (true) {
    var functionScoped = "var";
    let blockScoped = "let";
  }

  console.log(functionScoped); // "var"
  console.log(blockScoped); // ReferenceError
}

var belongs to the nearest function, so it escapes the if block. let and const belong to the nearest block, which makes them safer for loops and conditionals.

Hoisting and the temporal dead zone

console.log(a); // undefined
var a = 1;

console.log(b); // ReferenceError: Cannot access 'b' before initialization
let b = 2;

var is hoisted and initialised to undefined. let and const are also hoisted, but they are not initialised until execution reaches the declaration. The period between entering the scope and initialisation is the temporal dead zone.

Reassignment vs mutation

const user = { name: "Asha" };
user.name = "Ravi"; // allowed: mutating the object
user = {}; // TypeError: assignment to a constant binding

const locks the binding, not the value. Use const by default, switch to let only when the variable is reassigned, and avoid var in new code.

Common mistakes

• Saying const makes an object immutable. Use Object.freeze() for shallow immutability when you need it.
• Saying let and const are not hoisted. They are hoisted but remain uninitialised.
• Using var inside a loop with callbacks, then wondering why every callback sees the final index.